Ok, thank you all for replying even if the question seems annoying to you 'cause I'm pretty sure you already replied a lot of times to similar questions!!
Anyway, I understand that this is the normal behaviour..
What I would like to clarify is that I don't need and I don't want to change the classical folder and file-name structure: I was only asking if it's normal that even with more than one author the file-name only shows the first one (just like the folder, naturally).
Apart form that, again, I have no particular need to change that behaviour.. I already read the sticky thread explaining that.